Assembly of hollow torque transmitting sucker rods

ABSTRACT

An elongated drive string assembly includes a plurality of hollow sucker rods and connecting elements with an axis, connected together and between a drive head located at the surface of an oil well and a rotary pump located deep down in an oil well. Each hollow sucker rod has at least a first end including an internal female threaded surface engaging an external male threaded surface on a connecting element, such as a nipple. The first end also includes a torque shoulder, which engages a torque shoulder formed on the connecting element. The threads are frusto-conical, non-symmetrical threads with a differential diametral taper. The torque shoulders have a maximized mean diameter and cross-sectional area to resist storing reactive torque in the drive string. The nipple has a wall cross-section that increases towards the torque shoulders from each free end, to increase fatigue resistance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of U.S. Application Ser. No.09/679,359, filed Oct. 5, 2000 now abandoned, which claims the benefitof Argentina P99 01 06162, filed Dec. 3, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an elongated assembly of hollow, torquetransmitting pumping rods, used to selectively rotate a rotary pumplocated deep down hole in an oil well from a drive head located at thesurface of the oil well. A pumping rod assembly or sucker rod string issignificantly distinguished in the art by the fact that such a string isnot typically undergoing substantially free rotation like a drill pipestring, but rather is a true drive shaft that stores large amounts ofreactive torque due to its large length, typically between 1,500 to6,000 feet. The present invention comprises individual elements referredto herein as a “Hollow Sucker Rod” with at least a first end having afemale thread and a “Connecting Element” which may be a separate “NippleConnecting Element” with a pair of male threads or an integral malethread on a second, upset end of a Hollow Sucker Rod.

2. Description of the Related Art

Non-surging oil well extraction is normally achieved by means of pumpingsystems. The most common system uses an alternating pump located at thebottom of the well driven by a sucker rod string that connects thebottom of the well with the surface, where an alternating pumpingmachine to drive the string up and down is located. The sucker rods inthe prior art, therefore, were designed originally to simply reciprocateup and down, and were are manufactured to API Specification 11B usingsolid steel bars with an upset end and a threaded end, each thread beingof solid cylindrical section. The rods typically were connected one withthe other by means of a cylindrical threaded coupling. More efficientpumping is performed when an oil extracting progressive cavity pump(PCP), or like rotary down hole pump is used. Among other advantages,PCP pumping of oil allows for higher oil extraction rates, reducedfatigue loads, reduction in wear on the inside of production tubing, andthe ability to pump high viscosity and high solids component oils. PCPpumps are installed at the bottom of the well and driven from thesurface by an electric motor connected to a speed-reducing gearbox bymeans of a string of torque transmitting rods. Traditionally standardAPI sucker rods are used to drive PCP pumps notwithstanding the factthat these rods have not been designed to transmit torsional loads. Thetransmission of torque by means of sucker rod strings presents thefollowing disadvantages, i) low torque transmitting capacity, ii) highbackspin iii) big stiffness differential between the connection and therod body, all factors that enhance the possibility of fatigue failures.The reason for rupture on this type of conventional rod is failure dueto fatigue in the junction zone of the head of the rod with the body ofsame due to the difference in structural rigidity between both parts—thebody of the rod and the head of the rod.

For a given cross sectional area, torque transmission by a hollow rodwith an annular cross section is more efficient than with a narrower,solid circular cross section. With the above mentioned concept in mindthe prior art includes a hollow sucker rod that simply uses a standardAPI external cylindrical thread on a first end connector and an internalAPI thread on a second end connector, each connector being butt weldedto a pipe body, which creates significant and abrupt change in sectionbetween the pipe body and each connection body. (See Grade D HollowSucker Rod, CPMEC Brochure, undated). The problem of sucker rod stringbackspin, and details of a drive head at the surface of an oil well anda rotary pump deep down hole in an oil well operation, which is thespecific field of invention being addressed herein, can be found inMills (U.S. Pat. No. 5,551,510), which is incorporated herein byreference.

Various thread and shoulder arrangements are discussed in the prior artwith respect to joining together oil well drill pipe, well casing andtubing. See, for example, Pfeiffer et al. (U.S. Pat. No. 4, 955,644);Carstenson (U.S. Pat. No. 5,895,079), Gandy (U.S. Pat. No. 5,906,400),Mithoff (U.S. Pat. No. 262,086), Blose (U.S. Pat. No. 4,600,225), Watts(U.S. Pat. Nos. 5,427,418; 4,813,717; 4,750,761), Schock et al. (U.S.Pat. No. 6,030,004), and Hardy et al. (U.S. Pat. No. 3,054,628). TheWatts patents imply that a pre-1986 API standard for strings of casingand tubing was a straight thread, with a turned down collar and that hisimprovement comprised a flush joint tubular connection with both taperedthreads and a shoulder torque. Watts also refer to API standards fortubing and casing where triangular and buttress threads can be used witha torque shoulder. The 1990 patent to Pfeiffer et al, and the 1996patent to Carstensen et al, in contrast, refer to a more current APIstandard (truncated triangular thread, connection using a torqueshoulder) for strings of casing and tubing that appears to involvefrusto-conical threads and shoulders. Carstensen et al at col 7, line 9+include a discussion about how a particular conical gradient and lengthof a thread defines stress distribution results. Likewise, Pfeiffer etal at col 2, line 51+ say their threads are tapered and according to“API standards” with their improvement essentially only having to dowith transitional dimensions. Hence, the problem addressed by Pfeifferis an assembly of drill pipe sections where it apparently was criticalto use a compatible and standard non-differential thread according toAPI standards, and also with no incomplete threads and no torqueshoulder specification. The main features of the Pfeiffer thread appearto be symmetrical, truncated triangle threads (between 4 and 6 threadsper inch, 60° flank angle) and a thread height that is the same for themale and female thread (between 1.42 and 3.75 mm). Also, there isidentical nominal taper on male and female ends (between 0.125 and0.25). Schock et al. illustrate a particular tool joint for drill pipewhere the unexpected advantage for drill pipe applications derives fromtapered threads that significantly must be very coarse (3½ threads perinch) and have equal angle (75°) thread flanks and elliptical rootsurfaces.

However, the different problem of backspin inherent in the intermittentoperation of a sucker rod string when driving a PCP pump is notapparently addressed in any of these references. The design of theinvention was made with certain specific constraints and requirements inmind. First, the minimum diameter of the tubings on the inside of whichthe Hollow Rods must operate corresponds to API 2⅞″ tubing (innerdiameter=62 mm) and API 3½″ tubing (inner diameter=74.2 mm). The oilextraction flow rate must be up to 500 cubic meters per day, maximum oilflow speed must be 4 meters per second. The above-mentioned valuesstrongly restrict the geometry of the rods under design. Second, toensure a Hollow Rod with a high yield torque so that maximum torque istransmitted to the PCP pump without damage to the Hollow Rod string.Third, to minimize and distribute stresses in the threaded sections.This requirement is met by using a particular conical thread,differential taper, low thread height and a conical bore in the sectionsunder the threads. Fourth, the Hollow Sucker Rod must have good fatigueresistance. Fifth, to ensure low backspin, and high resistance to axialloads. Sixth, ease of make up and break out (assembly of mating threadedparts) must be ensured, and is by a tapered thread. Seventh, to ensurehigh resistance to unscrewing of the Hollow Sucker Rod due to backspin,or the counter-rotation of a sucker rod string when driving motor stopsrunning and the pump acts as a motor. Eighth, to ensure high resistanceto jump out of the Hollow Sucker Rod string (Hollow Rod parting at thethreaded sections) by means of adequate thread profile and reverse angleon the torque shoulder. Ninth, to minimize head loss of the fluids thatoccasionally can be pumped on the inside of the Hollow Sucker Rodthrough the added advantage of a conical bore on the nipple. Tenth, toensure connection sealabilty due to sealing at the torque shoulder, andalso due to diametrical interference at the threads. Eleventh, a threadprofile designed so as to optimize pipe wall thickness usage. Twelfth,to eliminate use of the welds due to susceptibility of welds to fatiguedamage, sulphide stress cracking damage and also the higher costs ofmanufacturing.

A first object of the present invention is to provide an assembly ofsucker pump rods and either separate threaded unions, or an integralunion at the second end of each sucker rod, to activate PCP and likerotary type pumps, capable of transmitting greater torque than the solidpump rods described in the API 11B Norm and also possessing good fatigueresistance. Additionally, the present invention seeks to define athreaded union for hollow rods that is significantly different from, andincompatible with, the standard for sucker rod assemblies as defined inthe API 11B Norm, yet still can easily be assembled. In fact themodified buttress thread is unique in that it is differential. Forexample, API Buttress Casing requires non-differential threads, with thetaper for both a pipe and a coupling being 0.625 inches/inch ofdiameter. Likewise, API 8r casing and API 8r tubing both also requirenon-differential threads, with the taper for both a pipe and a couplingbeing 0.625 inches/inch of diameter. Still further, each of API ButtressCasing, API 8r casing and API 8r tubing do not employ any manner oftorque shoulder.

A related object of the present invention is to provide an assembly ofpump rods and unions with lesser tendency to uncoupling of the unionswhenever “backspin” occurs, whether by accident or when intentionallyprovoked by the deactivation of the pump drive. The present inventionsurprisingly and significantly decreases the stored torsional energy ina sucker rod string. The stored energy in the string is inverselyproportional to the diameter of the rod, and is directly proportional tothe applied torque and the length of the string.

Another object of the invention is to provide for an assembly of suckerrods which are hollow and configured with a bore to permit passage oftools (sensors for control of the well) and/or allow interiorcirculation of fluids (injection of solvents and/or rust inhibitors).

SUMMARY OF THE INVENTION

The present invention addresses the foregoing needs in the art byproviding a new type of Hollow Sucker Rod consisting essentially of apipe central section, with or without an upset, with at least oneinternal or female conical thread at a first end having a threadvanishing on the inside of the rod and a conical external torqueshoulder. That first end is configured to engage a correspondingexternal or male thread that is differential and also to abut against aconical torque shoulder on either another rod with an externallythreaded integral Connecting Element as its second end, or one of theshoulders between the external threads of a separate Nipple ConnectingElement. If separate Nipple Connecting Elements are used, then thesucker rod second end is always the same as the first end. If separateNipple Connecting Elements are not used, then the sucker rod second endis configured with an upset end having a male conical thread adapted toengage the first end of another Hollow Sucker Rod.

A Nipple Connecting Element consists essentially of a centralcylindrical section with a pair of conical external torque shoulders.The torque shoulders have a maximized mean diameter and cross-sectionalarea to resist storing reactive torque in the drive string. The nipplepreferably also has a wall section that increases towards the torqueshoulders from each free end to increase fatigue resistance. In order tofurther optimize the stress distribution between the elements, aspecific type of thread with a differential taper is used. The overallconfiguration ensures high shear strength, lowered stress concentrationand a surprising resistance to storing reactive torque, which minimizesdangerous backspin when power to the sucker rod string is interrupted.

The Nipple Connecting Element member also has trapezoidal, non-symmetricmale threads at each end or extreme, separated by a pair of shoulderengaging elements, but that male thread is differential as to thediametral taper of the female thread on at least the first end of aHollow Sucker Rod. The threaded nipple and the rod can be joined with orwithout discontinuity of outer diameter. The ratio of the diameter ofthe union to the diameter of the rod may between 1 without discontinuityof diameters, to a maximum of 1.5. In this manner the mean value of theexternal diameter throughout the length of the string will always begreater to that of a solid rod with equivalent cross-sectional areamated to a conventional union means. Hence, for a given length of stringand cross-sectional area, resistance to “backspin” will be greater in anassembly according to the present invention. The dimensions of thenipple also may be defined with a conical inner bore proximate thelength of each threaded extreme, to further enhance an homogenousdistribution of tensions throughout the length of each thread and in thecentral body portion of the Nipple Connecting Element. In this way it ispossible to obtain a desired ratio of diameters of the threaded endingof the nipple with respect to the internal diameter, and a ratio ofoutside diameter of the nipple with respect to the internal diameter andan additional ratio between the external diameter of the nipple and thediameter of each threaded extreme.

In a first object of the present invention, the essential characteristicof a Hollow Sucker Rod is at least a first end of a tubular elementthreaded with a conical female thread which is configured as a ModifiedButtress or SEC thread and vanishes on the inside of the tubularelement, in combination with a conical frontal surface at an anglebetween 75° and 90°, known as a torque shoulder. The external diameterof the HSR 48×6 External Flush and the HSR 42×5 Upset embodimentscomprise a tubular rod body element away from the ends being 48.8 mm or42 mm and the external diameter of the tubular element in the upset endof a 42 mm rod being 50 mm. These dimensions are critical since suckerrods of that maximum diameter can fit within standard 2⅞ inch tubing (62mm inside diameter). For 3½ inch tubing (74.2 mm inside diameter) theHSR 48×6 Upset, with a diameter at the upset end of 60.6 mm, can be usedfor maximum advantage. The thread shape is trapezoidal andnon-symmetric, with a Diametrical taper in the threaded section. TheLength of threads on at least the first end of the tubular element areincomplete due to vanishing of thread on the inside of the tubularelement. There is an 83° angle (Beta) of the conical surface in thetorque shoulder as shown in FIG. 2A. There are radii at the inner andouter tips of the torque shoulder. At the end of the threaded section ashort cylindrical section on the inside of the threaded area transitionsthe threaded area to the bore of the tubular element.

In a first object of the present invention, the essential characteristicof a Nipple Connecting Element is a differential thread engagement oneither side of a central section that is externally cylindrical with alarger cross-sectional area in the vicinity of the torque shoulder forsurprisingly improved fatigue resistance. At either side of this centralsection external torque shoulders are located to mate with a torqueshoulder on a first end of a Hollow Sucker Rod. The mean diameter andtotal cross-sectional area of the torque shoulder is maximized, to allowmaximum torque handling.

In addition, either end of the nipple externally threaded is conical soto create a larger cross-sectional area in the vicinity of the torqueshoulder and thereby surprisingly improve fatigue resistance. To achievethis advantage a narrowing conical inner bore starts proximate the freeend of each threaded extreme and thereby defines an increasing wallthickness cross-section towards the central section of the nipple. Theexternal diameter of the central section of the nipple is 50 mm or 60.6mm and that central section may have a pair of machined diametricallyopposite flat surfaces, to be engaged by a wrench during connection makeup. The thread is a Modified Buttress thread, which creates adifferential due to slightly different amounts of diametral thread taperon the rod and on the nipple. The thread shape also is trapezoidal andnon-symmetric. All threads on the nipple are complete. A pair of conicalsurface act as torque shoulders with a conical frontal surface at anangle between 75° and 90°. There are radii at tips of the torqueshoulder, both at an inner corner and an outer corner. Preferably,conical bores under each threaded section of the nipple are connected bya cylindrical bore to create a larger cross-sectional area in theimmediate vicinity of the torque shoulder in order to surprisinglyimprove fatigue resistance.

The thread taper on the nipple and on the rod is slightly different(Differential Taper) to ensure optimal stress distribution. When theconnection is made up the corresponding torque shoulders on the rod andon the nipple bear against each other so that a seal is obtained thatprecludes the seepage of pressurized fluids from the outside of theconnection to the inside of said and vice-versa. This sealing effect isenhanced by the diametrical interference between the two mating threadedsections on the first end of the rod and on the nipple.

A better understanding of these and other objects, features, andadvantages of the present invention may be had be reference to thedrawings and to the accompanying description, in which there areillustrated and described different embodiments of the invention. All ofthe embodiments are considered exemplary of parts of a preferredassembly embodiment, since any one of the illustrated male ends willsuccessfully mate with any one of the illustrated female ends.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B, represent a Prior Art configuration of a conventionalsolid sucker rod as established in the API 11 B Norm specification.

FIGS. 2A, 2B and 2C respectively represent general configurations of aHollow Sucker Rod first end, a Nipple Connecting Element, and anassembly of both elements according to a first embodiment of theinvention, with a constant outer diameter.

FIG. 3A represents a general configuration of the assembly of a HollowSucker Rod having first and second female threaded ends and a NippleConnecting Element according to a second embodiment of the invention,with an upset end, or an enlarged outer diameter.

FIG. 3B represents a general configuration of the assembly of a HollowSucker Rod having a first female threaded end and a second end with amale threaded end according to a third embodiment of the invention, witha constant outer diameter.

FIGS. 4A, 4B and 4C respectively represent an axial section view, ashoulder detail view and a cross-section view along Line 4C—4C of aNipple Connecting Element having first and second male threaded ends,according to a fourth embodiment of the invention, styled Hollow Rod48×6 External Flush.

FIGS. 5A and 5B respectively represent an axial section view and ashoulder detail view of a Hollow Sucker Rod having a first femalethreaded end, according to the fourth embodiment of the invention.

FIGS. 6A, 6B and 6C respectively represent an axial section view, across-section view along Line 6B—6B and a shoulder detail view of aNipple Connecting Element having first and second male threaded ends,according to a fifth embodiment of the invention, styled Hollow Rod 42×5External Upset.

FIGS. 7A and 7B respectively represent an axial section view and ashoulder detail view of a Hollow Sucker Rod having a first femalethreaded end, according to the fifth embodiment of the invention.

FIGS. 8A, 8B and 8C respectively represent an axial section view, ashoulder detail view and a cross-section view along Line 8B—8B of aNipple Connecting Element having first and second male threaded ends,according to a sixth embodiment of the invention, styled Hollow Rod48.8×6 External Upset.

FIGS. 9A and 9B respectively represent an axial section view and ashoulder detail view of a Hollow Sucker Rod having a first femalethreaded end, according to the sixth embodiment of the invention.

FIG. 10A represents an axial section view and dimension detail view of afirst female threaded end on a Hollow Sucker Rod showing theconfiguration of a trapezoidal, non-symmetric thread profile that is aModified Buttress or SEC thread, according to the preferred embodimentsof the invention.

FIG. 10B represents an axial section view and dimension detail view of afirst male threaded end on a Nipple Connecting Element showing theconfiguration of a trapezoidal, non-symmetric thread profile that is aModified Buttress or SEC thread, according to the preferred embodimentsof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A represents a common solid sucker rod with its conventionalthreaded first end or head with a cylindrical-type male thread. A largediscontinuity between the head of the rod and the body of the rod caneasily be seen. Diameters DC and DV, respectively. FIG. 1B is aschematic of the assembly of that solid pump rod with a conventionalthreaded union or collar according to the API 11 B Norm.

FIGS. 2A-2C respectively represent general configurations of a HollowSucker Rod first end, a Nipple Connecting Element, and an assembly ofboth elements according to a first embodiment of the invention, with aconstant outer diameter. FIG. 2A gives references at the female extremeof the hollow rod according to the invention. It is also possible toobserve the frustro-conical shape threaded surface in the interior ofthe rod that diminishes in the internal diameter thereof. FIG. 2B givesreferences at the nipple or union according to the present invention.The external thread of frustro-conical shape and the presence of twotorque shoulders can also be seen. It is also possible to observe thevarying of the nipple inner bore diameter with conical shape labeled“Option A”, as indicated by a broken line, which in turn creates alarger cross-sectional area in the vicinity of the torque should andsurprisingly improves fatigue resistance.

FIG. 2C gives further references for the assembly of two hollow pumprods and one threaded union. It can be observed that the two femalethreads in the internal diameter of rod (3.a and 3.b) are joined to thecorresponding male ends (1 a and 1.b) and how torque shoulders (2 a and2.b) are part of nipple (2). The union between the corresponding maleand female extremes is accomplished by differential engagement of thefrustro-conical shape of the threads (5.a and 5.b). The fact that thethread shape is frustro-conical facilitates the initial setting of eachpiece and assembly of both parts. Shoulders located at the extreme freeend surfaces of the first and second ends of the hollow rods (4.a and4.b) engage, in the assembled position, against a pair of correspondingtorque shoulders formed on the nipple (2.a and 2.b). Said contact planesform a torque shoulder angle (angle “Beta” see FIG. 2A) with respect tothe axis of the rod, which angle being between 75° and 90° and mostpreferably being 83°.

FIG. 2B shows in general geometry references for a connecting element asa separate nipple and specifically defines outside diameter (DEN),internal diameter (DIN) and the start diameter of the torque shoulder(DHT). The connecting element for the invention is characterized by aratios of diameters according to the following table:

Range Diameter Ratios Min. Max. DHT/DEN 0.60 0.98 DIN/DEN 0.15 0.90DIN/DHT 0.25 0.92

FIG. 2B also illustrates, by the broken line, a conical bore option,Option A, for the nipple inner bore configuration, which is preferred.FIG. 2A shows the hollow rod in the union zone with an outside diameter(DEVU) and an internal diameter of the rod at the extreme surfaces ofthe first and second ends corresponding to the end of the thread (DIFR).It also shows the outside diameter of the hollow rod (DEV) labeled asDEVU=DEV, because there is no upset end acting as the union. The ratioof the maximum external diameter (DEVU), either of a separate connectorelement or the upset type end of integral connector element union, tothe external diameter of the rod (DEV), as illustrated at FIGS. 3A, 7Aand 9A, is maintained within the following range:$1 \leq \frac{DEVU}{DEV} \leq 1.5$

Hence for a maximum fixed diameter, the mean polar momentum of thehollow rod and connector string is greater than that for a solid pumprod of equal cross section diameter. Transmitted rotation moment ortorque is therefore greater in a hollow rod column than in a solid rodcolumn. This is also a determining factor in the resistance to the“backspin” phenomenon or counter-rotation of the rod string.Additionally, the ratio between the starting diameter of the torqueshoulder on the connecting element (DHT) and the internal diameter ofthe hollow rod at the thread free end (DIFR), is maintained, as follows:$1 \leq \frac{DIFR}{DHT} \leq 1.1$

FIG. 3A gives further references at the assembly in which the ratio ofthe maximum diameter of the union (DEVU) to the diameter of the body ofthe rod (DEV) is limited (1<DEVU/DEV≦1.5). FIG. 3B is a possibleconfiguration of the invention in which the female thread is machined onan upset first end of the rod, while the opposite or second end ismachined with a corresponding male thread, the two threads beingcomplementary but differential in diametral taper to each other. Thisconfiguration will be referred to as an upset rod, or as an integralunion version.

FIGS. 4-10, inclusive, relate to preferred embodiments where a HollowSucker Rod comprises at least a first end of a tubular element threadedwith a conical female thread which is configured as a Modified Buttressor SEC thread and which vanishes on the inside of the tubular element,in combination with a torque shoulder angle (Beta) of between 75° and90°. The external diameter of the tubular element away from the endsbeing either 42 mm or 48.8 mm and the external diameter of the tubularelement in the upset end, if present, being either 50 or 60.6 mm.

FIGS. 4A, 4B and 4C respectively represent an axial section view, ashoulder detail view and a cross-section view along Line 4C—4C of aNipple Connecting Element 402 with a flat 406 having first and secondmale threaded ends, 401 and 401.b, according to a fourth embodiment ofthe invention, styled Hollow Rod 48×6 External Flush. In FIG. 4A thevalues are a Modified SEC thread 405.b, 8 threads per inch; DEN=48.8 mm;DIN=20 mm with an expansion to 26 mm over a length of 44 mm to theextreme end; DHT=39 mm; Beta=83°; overall length=158 mm; threadlength=46 mm and central section length=50 mm. The shoulder detail 402 ain FIG. 4B begins 4.61 mm after the thread, has an inner radius of 1.4mm and an outer shoulder radius of 0.5 mm.

FIGS. 5A and 5B respectively represent an axial section view and ashoulder detail view of a Hollow Sucker Rod 403 having a first femalethreaded end 403 a, according to the fourth embodiment of the invention.In FIG. 5A the values are a Modified SEC thread 405 a, 8 threads perinch; DEV=48.8 mm; DIFR=41.4 mm; DIV=37 mm; Beta=83°. The shoulderdetail 404 a in FIG. 5B has a 30° transition at the thread and extends4.5 mm; has an inner radius of 0.8 mm and an outer shoulder radius of0.5 mm.

FIGS. 6A, 6B and 6C respectively represent an axial section view, across-section view along Line 6B—6B and a shoulder detail view of aNipple Connecting Element 502 with flat 506 and having first and secondmale threaded ends, 501 a and 501.b, according to a fifth embodiment ofthe invention, styled Hollow Rod 42×5 External Upset. In FIG. 6A thevalues are a Modified SEC thread 505.b, 8 threads per inch; DEN=50 mm;DIN=17 mm with an expansion to 25.3 mm over a length of 44 mm to theextreme end; DHT=38.6 mm; Beta=83°; overall length=158 mm; threadlength=46 mm and central section length=50 mm. The shoulder detail 502 ain FIG. 6C begins 4.61 mm after the thread, has an inner radius of 1.4mm and an outer shoulder radius of 0.5 mm.

FIGS. 7A and 7B respectively represent an axial section view and ashoulder detail view of a Hollow Sucker Rod 503 having a first femalethreaded end 503 a, according to the fifth embodiment of the invention.In FIG. 7A the values are a Modified SEC thread 505 a, 8 threads perinch; DEVU ranging from 50 mm to DEV=42 mm; DIFR=41 mm; DIV=36.4 mm witha transition at 15° to 30 mm starting at 55 mm from the free end andback to 32 mm over a maximum length of 150 mm; Beta=83°. The shoulderdetail 504 a in FIG. 7B has a 30° transition at the thread and extends4.5 mm; has an inner radius of 0.8 mm and an outer shoulder radius of0.5 mm.

FIGS. 8A, 8B and 8C respectively represent an axial section view, ashoulder detail view and a cross-section view along Line 8B—8B of aNipple Connecting Element 602 with flat 606 and having first and secondmale threaded ends, 601 a and 601.b, according to a sixth embodiment ofthe invention, styled Hollow Rod 48.8×6 External Upset. In FIG. 8A thevalues are a Modified SEC thread 605.b, 8 threads per inch; DEN=60.6 mm;DIN=20 mm with an expansion to 33.6 mm over a length of 44 mm to theextreme end; DHT=47 mm; Beta=83°; overall length=158 mm; threadlength=46 mm and central section length=50 mm. The shoulder detail 602 ain FIG. 8C begins 4.61 mm after the thread, has an inner radius of 1.4mm and an outer shoulder radius of 0.5 mm.

FIGS. 9A and 9B respectively represent an axial section view and ashoulder detail view of a Hollow Sucker Rod 603 having a first femalethreaded end 603 a, according to the sixth embodiment of the invention.In FIG. 9A the values are a Modified SEC thread 605 a, 8 threads perinch; DEVU ranging from 60.6 mm to DEV=48.8 mm; DIFR=49.4 mm; DIV=44.6mm with a transition at 15° to 30 mm starting at 55 mm from the free endand back to 35.4 mm over a maximum length of 150 mm; Beta=83°. Theshoulder detail 604 a in FIG. 9B has a 30° transition at the thread andextends 4.5 mm; has an inner radius of 0.8 mm and an outer shoulderradius of 0.5 mm.

FIG. 10A represents an axial section view and dimension detail view of afirst female threaded end on a Hollow Sucker Rod showing theconfiguration of a trapezoidal, non-symmetric thread profile that is aModified Buttress or SEC thread, according to the rod first endpreferred embodiment. The female thread shape of each Hollow Sucker Rodis trapezoidal and non-symmetric and is incomplete. The thread pitch is8 threads per inch. The thread height is 1.016+0/−0.051 mm. TheDiametrical taper in the threaded section is 0.1 mm/mm. The Length ofthreads on at least the first end of the tubular element is 44 mm., withpart of the threads being incomplete due to vanishing of thread on theinside of the tubular element. The thread taper angle is 2° 51′ 45″; thetooth inner surface is 1.46 mm and the teeth spacing is 1.715 mm; theleading edge has a 4° taper or load flank angle and an inner radius of0.152 mm while the trailing edge has a 8° taper and a larger innerradius of 0.558 mm. At the end of the threaded section a shortcylindrical section on the inside of the threaded area transitions thethreaded area to the bore of the hollow tubular element.

FIG. 10B represents an axial section view and dimension detail view of afirst male threaded end on a Nipple Connecting Element showing theconfiguration of a trapezoidal, non-symmetric thread profile that is aModified Buttress or SEC thread, according to the nipple first or secondend preferred embodiment. The external diameter of the central sectionof each Nipple Connecting Element is 50 mm or 60.6 mm and the centralsection can present a pair of machined diametrically opposite flatsurfaces, to be engaged by a wrench during connection make up. The malethread is a Modified Buttress thread and is complete across both ends ofthe nipple. The threaded section pitch is 8 threads per inch. The threadheight lies between 1.016+0.051/−0 mm. The diametrical thread taper inthe threaded area is 0.0976 mm/mm. The thread shape is trapezoidal andnon-symmetric. The length of threads on each extreme of the nipple is 46mm. All threads on the nipple are complete. The angle of the conicalsurface in the torque shoulder (Beta) is 83°. The radius at the tips ofthe torque shoulder is 1.4 mm for the internal radius and 0.5 mm for theexternal radius. There are preferred conical bores under each threadedsection of the nipple, which are connected by a cylindrical bore. Thethread taper angle is 2° 47′ 46″; the tooth inner surface is 1.587 mmand the teeth spacing is 1.588 mm; the trailing edge has a 4° taper orload flank angle and an outer radius of 0.152 mm while the leading edgehas a 8° taper and a larger outer radius of 0.558 mm.

Hence, for all preferred embodiments, there is a diametral ordifferential taper. For example the rod first end taper is 0.1inches/inch, while the corresponding taper of the either nipple end is0.0976 inches/inch. For all preferred embodiments, the angle of theconical surface in the torque shoulder (Beta) is preferably 83°. Theradiuses at the tips of the torque shoulder are 0.8 mm for the internalradius and 0.5 mm for the external radius.

Likewise, for all preferred embodiments, the Connecting Element has acentral section that is externally cylindrical. Close to the outerdiameter of this central section external torque shoulders are locatedto mate with the torque shoulder on a first end of a Hollow Sucker Rod.Both extremes of a nipple are conical and externally threaded, and aconical inner bore proximate the length of each threaded extreme createsan advantageous combination of structure, to ensure an increasingcross-section of the nipple from each free end of the nipple towards thecentral section, and the torque shoulder locations.

While preferred embodiments of our invention have been shown anddescribed, the invention is to be solely limited by the scope of theappended claims.

We claim:
 1. An elongated drive string assembly comprising a pluralityof hollow sucker rods and connecting elements with an axis, connectedtogether and between a drive head located at the surface of an oil welland a rotary pump located deep down the oil well, wherein each hollowsucker rod has at least a first end comprising an internal femalethreaded surface engaging an external male threaded surface on aconnecting element, wherein said threads are frusto-conical andnon-symmetrical, but differential in diametral taper to each other; thefirst end of each hollow sucker rod further comprising an annular torqueshoulder engaging an annular torque shoulder on the connecting element,and being characterized in that, for an outside diameter of theconnecting element (DEN), an internal diameter (DIN) of the connectingelement, and a starting diameter of the torque shoulder on theconnecting element (DHT), the following ratios are maintained: RangeDiameter Ratios Min. Max. DHT/DEN 0.60 0.98 DIN/DEN 0.15 0.90 DIN/DHT0.25  0.92.


2. An elongated drive string assembly according to claim 1, wherein thedifferential threads are each non-symmetrical truncated trapezoids inthread shape; the thread is complete on the connecting element andincomplete on the first end of the hollow rod; the torque shoulder onthe connecting element makes an angle with the axis of the rod which isbetween 75° and 90°, and being characterized in that the ratio of theoutside diameter of the connecting element (DEVU) to the outsidediameter of the hollow rod (DEV) is maintained within the followingrange: $1 \leq \frac{DEVU}{DEV} \leq {1.5.}$


3. An elongated drive string assembly according to claim 2, wherein theratio between the internal diameter of the hollow rod at the first end(DIFR) and the starting diameter of the torque shoulder on theconnecting element (DHT) is maintained within the following range:$1 \leq \frac{DIFR}{DHT} \leq {1.1.}$


4. An elongated drive string assembly according to claim 1, wherein eachconnecting element is a separate nipple connector element having malethreads on each end separated by a central section defining a pair oftorque shoulders wherein the differential threads are eachnon-symmetrical truncated trapezoids in thread shape; the thread iscomplete on the nipple and incomplete on the first end of each hollowrod; each torque shoulder in said pair makes an angle with the axis ofthe rod which is between 75° and 90°, and being characterized in thatthe ratio of the outside diameter of the connecting element (DEVU) tothe outside diameter of the hollow rod (DEV) is maintained within thefollowing range: $1 \leq \frac{DEVU}{DEV} \leq {1.5.}$


5. An elongated drive string assembly according to claim 4, wherein theratio between the internal diameter of the hollow rod at the first end(DIFR) and the starting diameter of the torque shoulder on the nippleconnector element (DHT) is maintained within the following range:$1 \leq \frac{DIFR}{DHT} \leq {1.1.}$


6. An elongated drive string assembly according to claim 1, wherein eachconnecting element is an integral male thread on the second end of eachhollow rod wherein the differential threads are each non-symmetricaltruncated trapezoids in thread shape; the thread is complete on thesecond end and incomplete on the first end of each hollow rod; eachtorque shoulder on a second end makes an angle with the axis of the rodwhich is between 75° and 90°, and being characterized in that the ratioof the outside diameter of the connecting element (DEVU) to the outsidediameter of the hollow rod (DEV) is maintained within the followingrange: $1 \leq \frac{DEVU}{DEV} \leq {1.5.}$


7. An elongated drive string assembly according to claim 6, wherein theratio between the internal diameter of the hollow rod at the first end(DIFR) and the starting diameter of the torque shoulder on the hollowrod second end (DHT) is maintained within the following range:$1 \leq \frac{DIFR}{DHT} \leq {1.1.}$


8. An elongated drive string assembly according to claim 1, wherein thedifferential threads are each non-symmetrical truncated trapezoids inthread shape; each torque shoulder makes an angle with the axis of therod which is substantially 83°; the connector element is a separatenipple connector element having substantially conical threads with malethreads on each end separated by a central section defining a pair oftorque shoulders, and an internal cylindrical bore within the centralsection communicates with a conical inner bore that is proximate to eachthreaded end so as to define an increasing cross-section of the nipplefrom each end of the nipple towards the central section and the torqueshoulder locations.
 9. An elongated drive string assembly according toclaim 8, wherein the differential threads are modified buttress threads,have a pitch of substantially eight threads per inch, and the nippleconnector element male threads have a diametral taper in inches per inchof diameter of substantially 0.0976, and the hollow rod female threadshave a diametral taper in inches per inch of diameter of substantially0.1.
 10. A hollow sucker rod adapted to engage a connecting elementalong an axis, wherein the hollow sucker rod has at least a first endcomprising an internal female threaded surface adapted to engage anexternal male threaded surface of a connecting element, which iscomplementary but differential in diametral taper to the female threadedsurface; said first end of the hollow sucker rod further comprising anannular torque shoulder adapted to engage an annular torque shoulder ofa connecting element, and being characterized in that, for an outsidediameter of a connecting element (DEN), an internal diameter (DIN) of aconnecting element, and a starting diameter of the torque shoulder of aconnecting element (DHT), the following ratios are maintained: RangeDiameter Ratios Min. Max. DHT/DEN 0.60 0.98 DIN/DEN 0.15 0.90 DIN/DHT0.25  0.92.


11. A hollow sucker rod according to claim 10, wherein the differentialthread is a non-symmetrical truncated trapezoid in thread shape; thethread is incomplete on the first end of the hollow rod; the torqueshoulder at the first end makes an angle with the axis which is between75° and 90°, and being characterized in that the ratio of the outsidediameter of a connecting element (DEVU) to the outside diameter of thehollow rod (DEV) is maintained within the following range:$1 \leq \frac{DEVU}{DEV} \leq {1.5.}$


12. A hollow sucker rod according to claim 11, wherein the ratio betweenthe internal diameter of the hollow rod at the first end (DIFR) and thestarting diameter of the torque shoulder on the connecting element (DHT)is maintained within the following range:$1 \leq \frac{DIFR}{DHT} \leq {1.1.}$


13. A hollow sucker rod according to claim 12, wherein the differentialthread is a modified buttress thread, has a pitch of substantially eightthreads per inch, and the hollow rod female thread has a diametral taperin inches per inch of diameter of substantially 0.1 that is adapted toengage a connecting element with a diametral taper in inches per inch ofdiameter of substantially 0.0976.
 14. A connecting element adapted toengage a hollow sucker rod along an axis, wherein an external malethreaded surface on the connecting element is adapted to engage a hollowsucker rod first end comprising an internal female threaded surfacewhich is complementary but differential in diametral taper to the malethreaded surface; the connecting element further comprising an annulartorque shoulder adapted to engage an annular torque shoulder on thefirst end of a hollow sucker rod, and being characterized in that, foran outside diameter of the connecting element (DEN), an internaldiameter (DIN) of the connecting element, and a starting diameter of thetorque shoulder of the connecting element (DHT), the following ratiosare maintained: Range Diameter Ratios Min. Max. DHT/DEN 0.60 0.98DIN/DEN 0.15 0.90 DIN/DHT 0.25  0.92.


15. A connecting element according to claim 14, wherein the connectingelement is a separate nipple connector element having male threads oneach end separated by a central section defining a pair of torqueshoulders; the differential threads are each non-symmetrical truncatedtrapezoids in thread shape; the thread is complete on the nipple; eachtorque shoulder in said pair makes an angle with the axis which isbetween 75° and 90°, and being characterized in that the ratio of theoutside diameter of the connecting element (DEVU) to the outsidediameter of a hollow rod (DEV) is maintained within the following range:$1 \leq \frac{DEVU}{DEV} \leq {1.5.}$


16. A connecting element according to claim 15, wherein the ratiobetween the internal diameter of a hollow rod at the first end (DIFR)and the starting diameter of the torque shoulder on the connectingelement (DHT) is maintained within the following range:$1 \leq \frac{DIFR}{DHT} \leq {1.1.}$


17. A connecting element according to claim 14, wherein the connectingelement is an integral male thread on the second end of a hollow suckerrod, wherein the differential threads are each non-symmetrical truncatedtrapezoids in thread shape; the thread is complete on the second end andincomplete on the first end of a hollow sucker rod; the torque shoulderof the connecting element makes an angle with the axis of a rod which isbetween 75° and 90°, and is characterized in that the ratio of theoutside diameter of the connecting element (DEVU) to the outsidediameter of a hollow rod (DEV) is maintained within the following range:$1 \leq \frac{DEVU}{DEV} \leq {1.5.}$


18. A connecting element according to claim 17, wherein the ratiobetween the internal diameter of a hollow rod at the first end (DIFR)and the starting diameter of the torque shoulder on the connectingelement (DHT) is maintained within the following range:$1 \leq \frac{DIFR}{DHT} \leq {1.1.}$


19. A connecting element according to claim 14, wherein the differentialthread is a non-symmetrical truncated trapezoid in thread shape; thetorque shoulder of the connecting element makes an angle with the axisof substantially 83°; and the connecting element is a separate nippleconnector element having substantially conical threads with male threadson each end separated by a central section defining a pair of torqueshoulders, an internal cylindrical bore within the central sectioncommunicates with a conical inner bore proximate each threaded end, soas to define an increasing cross-section of the nipple from each end ofthe nipple towards the central section and the torque shoulderlocations.
 20. A connecting element according to claim 19, wherein thedifferential thread is a modified buttress thread, has a pitch ofsubstantially eight threads per inch, and each of the nipple connectorelement male threads has a diametral taper in inches per inch ofdiameter of substantially 0.0976 and is adapted to engage the femalethreaded surface of a hollow sucker rod, which has a diametral taper ininches per inch of diameter of substantially 0.1.